1. Field of the Invention
The present invention relates to an image forming apparatus that has a charging member for contacting an image bearing member to charge it, in which a peripheral velocity difference is formed between the image bearing member and the charging member.
2. Related Background Art
a) Transfer-type Image Forming Apparatus
A transfer-type image forming apparatus is an apparatus with a system and configuration for causing a first image bearing member such as an electrophotographic photosensitive member and an electrostatic recording dielectric to form and bear a transferable image in an appropriate image forming process such as an electrophotographic process and an electrostatic recording process and to transfer the transferable image to a second image bearing member and repeatedly using the first image bearing member for image formation.
For example, an image forming apparatus such as a copying machine/laser beam printer utilizing a transfer electrophotographic process basically has an electrophotographic photosensitive member as a first image bearing member, which is generally a rotary drum type, charging means for uniformly charging the surface of the rotary photosensitive member in predetermined polarity and potential, image exposing means for forming an electrostatic latent image (electronic latent image) on a charging processing surface of the rotary photosensitive member, developing means for developing the electrostatic latent image as a toner image, transferring means for transferring the toner image from the surface of the photosensitive member to transfer paper as a second image bearing member, fixing means for fixing the toner image transferred to the transfer paper side as a permanently fixed image and photosensitive member cleaning means (cleaner) for removing transfer residual toner from the surface of the rotary photosensitive member, from which the toner image has been transferred to the transfer paper side, to clean the surface of the photosensitive member.
The transfer paper that was subjected to an image fixing processing by the fixing means is discharged as a product on which an image is formed (copy, print). The surface of the photosensitive member cleaned by the cleaning means serves image formation repeatedly.
b) Contact Charging Means
As charging means for uniformly charging a surface of a photosensitive member in predetermined polarity and potential, a corona charger has been conventionally used.
The corona charger is disposed non-contactingly opposing a charged member and exposes the surface of the charged member to a corona shower discharged from the corona charger, to which a high pressure is applied, to charge the surface in predetermined polarity and potential.
Instead, a contact electrifying apparatus has been put to practical use in recent years. This causes a conductive charging member of a roller type (charging roller), a fur brush type, a magnetic brush type, a blade type and the like to contact a photosensitive member as a charged member and applies a predetermined charging bias to it to charge the surface of the photosensitive member in predetermined polarity and potential. The contact electrifying apparatus has advantages in terms of low ozone, low electric power and the like compared with the corona charger.
As a method of applying a charging bias to a contact charging member, there are a DC bias method for applying only a direct current bias and an AC bias method for superimposing a direct current bias on an alternating current bias to apply the superimposed bias.
Two types of charging systems, namely a corona charging system and a contact injection charging system (direct charging system) are mixed in a charging mechanism (mechanism of charging, charging principle) of contact charging. Respective properties emerge depending on which one is more predominant.
The corona charging system is a system for using a discharge phenomenon such as the corona discharge that occurs in a microscopic gap between a contact charging member and a charged member to charge the charged member with a product of discharge. This corona charging system produces a small amount of ozone although it is significantly less than that in the case of the corona charger.
The contact injection charging system is a system in which a charge is directly injected in a charged member from a contact charging member, whereby the surface of the charged member is charged. This is also called direct charging or injection charging.
In U.S. Pat. No. 5,809,379 and the like, there is proposed a method of injecting a charge via a charging roller, a charging brush, a magnetic brush formed of conductive magnetic particles, or a contact charging member or the like of conductive particulates or the like carried on a roller to perform the contact injection charging.
Since the contact injection charging does not use the discharge phenomenon, a voltage required for charging is equivalent to only a desired surface potential of a photosensitive member and does not produce ozone. Thus, it is an ozoneless and low power charging method.
Since the contact injection charging does not use the discharge phenomenon in a microscopic gap, its charging capability and charging uniformity largely depend on a probability of contact between a surface of a photosensitive member and a charging member. It is a general practice to give a velocity difference between the surface of the photosensitive member and the contact charging member in order to increase this probability of contact.
c) Cleanerless (Cleanerless Process, Toner Recycle Process)
For the transfer-type image forming apparatus, there is proposed a method of causing an electrifying apparatus or a developing apparatus to perform cleaning (removal) of residual toner after transfer (transfer residual toner) on a photosensitive member without specifically providing a special purpose cleaner for compactness of the apparatus, reduction of costs, ecology and the like (cleaning simultaneous with charging or cleaning simultaneous with development).
That is, in a general transfer-type image forming apparatus, transfer residual toner remaining on a photosensitive member after transfer is removed from the surface of the photosensitive member by a special purpose cleaner to be waste toner. However, it is preferable from the viewpoint of maintenance or ecology that the waste toner is not produced.
Thus, there has been developed a cleanerless image forming apparatus with a configuration in which a cleaner is eliminated and a transfer residual toner on a photosensitive member after transfer is removed from the surface of the photosensitive member through the cleaning simultaneous with development by a developing apparatus and collected in the developing apparatus for reusing. That is, the developing apparatus also works as a cleaner for collecting transfer residual toner remaining on the photosensitive member after a toner image is transferred to a transferring material.
The cleaning simultaneous with development is a method of collecting toner remaining on a photosensitive member after transfer by a fog removing bias (a fog removing potential difference Vback that is a potential difference between a direct current voltage to be applied to a developing apparatus and a surface potential of the photosensitive member) at the time of development in the next and subsequent processes. According to this method, since the transfer residual toner is collected in the developing apparatus and reused in the next and subsequent processes, waste toner is not discharged. In addition, since it is cleanerless, there is a significant advantage in terms of space and compactness, reduction of costs and the like of an image forming apparatus can be realized.
d) Contact Charging, Transfer and Cleanerless Image Forming Apparatus
In a transfer and cleanerless image forming apparatus, if a contact electrifying apparatus is adopted as charging means of a photosensitive member as an image bearing member, a positive ghost due to poor collection of transfer residual toner in a developing apparatus can be suppressed by a scattering effect of the transfer residual toner by a charging member that contacts the photosensitive member. Since it is cleanerless, there are such advantages that there is no damage on the surface of the photosensitive member due to a cleaning member contactingly sliding on the surface of the photosensitive member.
In magnetic brush charging using a magnetic brush charging member (magnetic brush charger) having a magnetic brush portion, which is formed in a brush shape by magnetically binding conductive magnetic particles, as a charging member, the magnetic brush charging member is unlikely to suffer from an effect of toner mixture compared with other charging members such as a conductive rubber roller and a fixed or rotary fur brush and is preferably used in a cleanerless process.
In addition, in a particle charging method for contactingly charging a photosensitive member via nonmagnetic and conductive particulates (charging particles, charging accelerating particles) carried on a roller disclosed in, for example, U.S. Pat. Nos. 6,081,681, 6,128,456 and 6,134,407, Japanese Patent Application Laid-Open No. 10-307458 and the like, there is described a method of externally adding conductive particulates to toner of a developing apparatus in advance and developing the conductive particulates with the toner, thereby supplying the conductive particulates to the roller to realize a cleanerless process.
In the contact injection charging method, due to a characteristic that a charge is only injected in a part which a charging member of an image bearing member contacts, when the charging member is contaminated by toner, its added agent or the like, contact of the charging member and the image bearing member is prevented and charge injection in the image bearing member is hindered. As a result, voltage drop or charging unevenness is caused. The former can be solved by a method of measuring a potential of the image bearing member and controlling a voltage to be applied to the charging member such that a desired potential can be obtained. However, the latter, that is, the charging unevenness due to fall of charging capability cannot be solved. This charging unevenness is caused because charging nonuniformity of image bearing property and a mechanical accuracy error of the charging member or the image bearing member and the like become conspicuous due to decrease of the charging capability of the charging member. This is most conspicuous in a cleanerless process in which a large amount of toner is mixed into the charging member.
In order to compensate for this disadvantage, the capability of the charging member must be increased by making a velocity difference between the image bearing member and the charging member sufficiently large and increasing the contact probability. Thus, in setting the velocity difference, it is necessary to set it a little larger considering contamination of the charging member in advance. On the other hand, since an amount of wear of the image bearing member and the velocity difference of the charging member are substantially proportionate, the longer the life of the charging member the larger the velocity difference becomes by itself, whereby the amount of wear of the image bearing member increases and the life of the image bearing member is shortened.
As a method of decreasing wear of a photosensitive member, which is an image bearing member, in the image forming apparatus of the contact charging method, Japanese Patent Application Laid-Open No. 9-96948 proposes changing a velocity difference of a charging member only when an image is not formed. However, it does not take wear during image formation into account.
The present invention has been devised in view of the above drawbacks, and it is an object of the present invention to provide an image forming apparatus that reduces wear of an image bearing member by a charging member.
It is another object of the present invention to provide an image forming apparatus that can realize an extended life of an image bearing member.
It is another object of the present invention to provide an image forming apparatus in which decrease of a charging performance of a charging member is prevented.
It is another object of the present invention to provide an image forming apparatus that can satisfactorily perform charging even if a charging member is contaminated.
It is yet another object of the present invention to provide an image forming apparatus in which a peripheral velocity difference is provided between a charging member and an image bearing member.
Other objects and characteristics of the present invention will be more apparent by reading the following detailed description with reference to accompanying drawings.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the present invention.